This invention relates to vapor phase equilibration and more particularly, to vapor phase equilibration between two solids, one of which solids is a crystalline solid typically used in fabricating optical waveguiding members.
Optical waveguiding layers on either electro- or acousto-optical materials are attractive in a variety of discrete and integrated optical device applications that incorporate modulation, deflection and/or scanning of light. It is well known that an optical waveguiding slab structure requires that the refractive index of its thin surface region exceed that of its bulk. This has been accomplished in the past by an assortment of deposition techniques yielding a higher refractive index film adjacent to a lower index substrate. However, in general, deposition techniques have suffered from limited optical quality, homogeneity, and reproducibility, as well as a difficulty in modifying the refractive index gradient magnitude and profile.
In recent years, interest in forming active optical waveguiding structures has been stimulated by the need for optical modulators and deflectors, as well as for more efficient harmonic generators. To date, for device applications, experimental emphasis has been placed on those waveguides formed within, or on, single crystalline materials such as lithium niobates: J. M. Hammer and W. Phillips, Appl. Phys. Lett., 24, 545 (1974) I. P. Kaminow and J. R. Carruthers, Appl. Phys., Lett., 22, 326 (1973); J. R. Carruthers, I. P. Kaminow and L. W. Stulz, Appl. Opt., 13, 2333 (1974); I. P. Kaminow, J. R. Carruthers, E. H. Turner, and L. W. Stultz, Appl. Phys. Lett., 22, 540 (1973); U.S. Pat. Nos. 3,837,827 and 3,911,176. Single crystalline materials such as the garnets have also been emphasized: P. K. Tien, R. J. Martin, S. L. Blank, S. H. Wemple, and L. J. Varnerin, Appl. Phys. Lett., 21, 207 (1972); P. K. Tien, R. J. Martin, R. Wolfe, R. C. LeCraw and S. L. Blank, Appl. Phys. Lett., 21, 397 (1972); and P. K. Tient and D. P. Schinke, J. Appl. Phys., 45, 2059 (1974).
Promising results have been obtained by forming a thin layer within a substrate such as, for example, by diffusion, ion exchange, or ion implantation. See, for example, the extensive reference list contained in P. K. Tien and A. A. Ballman, J. Vac. Technol., 12, 892 (1975). Vacuum out diffusion such as reported in the lithium niobate articles cited above produces a nonstoichiometric waveguide layer characterized by a large defect concentration of lattice vacancies and the formation of lithium deficient phases at, or near, the surface. Metal ion, in-diffused waveguides such as reported in the above cited articles on garnets and in R. V. Schmidt and I. P. Kaminow, Appl. Phys. Lett., 25, 458 (1974) and in J. Noda, T. Saku and N. Uchida, Appl. Phys. Lett., 25, 308 (1974) develop equivalent defect concentrations of lattice vacancies and usually leave a residual layer of nondiffused oxide on the surface.
Neither approach offers sufficient flexibility in adjusting either the magnitude of the refractive index change or the shape of the refractive index profile to provide reproducible results of the degree of stability required for stabilization and interchangeability of like components in fine tuned integrated optical circuitry.
The present invention expands upon techniques developed by the inventor and reported in "Intrinsic and Extrinsic Nonstoichiometry in the Lead Zirconate-Titanate System", Ph. D. Thesis, Univ. of Calif., Berkeley, 1972 (LBL-880); "Intrinsic Nonstoichiometry in the Lead Zirconate-Lead Titanate System Determined by Knudsen Effusion," J. Appl. Phys., 44, 5227 (1973); and "Novel Uses of the Thermo-Microbalance in the Determination of Nonstoichiometry in Complex Oxide Systems," J. Vac. Sci. Technol., 11, 434 (1974) to solve the reproducibility problem in optical waveguides and to control the magnitude and profile of the optical index of refraction therein; and, to achieve mass transport under vapor phase equilibrium conditions between solids having a common vapor phase whose vapor pressure is less than about 10.sup.-4 1 atm by redesigning the crucibles used in the inventor's previous techniques.
Another difficulty is that even in commercially supplied crystals to sample-to sample variances have heretofore precluded standardization of index of refraction for a given crystalline material. Furthermore, a need exists for a method of neutralizing optical damage in crystalline materials and in waveguide layers of crystalline members.